Pain in lower left leg while walking?

Area of pain. often details the reason. Knee, ankle pain would obviously imply a joint problem. Pain/fatigue in the calves that increases with increasing exercise implies a circulatory problem. Pain with swelling in one lower leg is actually an emergency since the circulatory problem can be a blood clot that could come loose and go to the lung. This would be pain ALL the Time and generally a LOT of swelling.
Not a lot of info... With the only information being "pain while walking" the list of possible causes is extremely long. I would suggest if this has been going on for some time that you speak with a physician who will get a much more thorough history and be able to do an exam. The duration of pain, location of pain, severity, history of injury, quality of pain, duration of symptoms, medications, etc are all needed.
Several causes. Arterial, venous, Muscular, Neurological Needs physical exam to go along with complete history. Commonly peripheral arterial obstruction in a smoker!